1. Field of the Invention
This invention relates to an electrophotographic process, electrostatic printing process or the like, and more particularly, to a copying process which comprises developing an electric latent image with toner particle, transferring the developed image to a paper and fixing the transferred image mainly by means of pressure.
2. Description of the Prior Art
Heretofore, various electrophotographic processes have been known. For example, U.S. Pat. Nos. 2,297,691, 2,576,047 and 3,081,698, and Japanese Patent Publication Nos. 23910/1967 and 24784/1968 disclose electrophotographic processes. In general, these electrophotographic processes utilize a photoconductive material as a photosensitive material and comprise forming an electric latent image on a photosensitive member including the photoconductive material by various image-forming means, developing the latent image with a toner, and if desired transferring the toner image to a transferring material such as paper, and further fixing the transferred image by, for example, heating, pressing or applying a solvent vapor so that a copy is obtained.
Also, these are known various methods for visualizing an electric latent image with a toner, and they includes many developing methods, for example, magnetic brush method disclosed in U.S. Pat. No. 2,874,063, cascade developing method proposed in U.S. Pat. No. 2,618,552, powder cloud method disclosed in U.S. Pat. No. 2,221,776, fur brush developing method, liquid developing method and the like. The toner used in these developing methods is fine powder prepared by dispersing a coloring material such as a dye and pigment in natural or synthetic resin. Further, it is known that a developing powder containing a third substance added for various purposes is used as the toner.
The developed toner image is transferred to a transferring material such as paper if desired, and then fixed. As the method for fixing the toner image, there are known a method of heat-melting the toner of the image by means of a heater or heat roller to cause it to deposit and solidify on a substrate, a method of softening or dissolving the toner with an organic solvent to fix it on a substrate, a method of pressing the toner onto a substrate to effect the fixation, and the like.
These toners are prepared from different material selected in accordance with the kind of fixing method to be applied. Therefore, in general it is not possible to utilize the toner suitable for one fixing method as that for the other fixing methods. Particularly, the toner suitable for the heat-melting fixing method using a heater as conventionally used in wide range can be hardly utilized in the heat roller fixing method, solvent fixing method, pressure fixing method and the like. Consequently, toners suitable for various fixing methods are studied and developed.
Among those fixing methods, the method using a solvent is excellent in that a toner can be perfectly fixed by a small quantity of energy, but has such a drawback that the solvent vapor escapes from the copying machine to pollute the surroundings. Further, the fixing method using heat which is the most common fixing method is advantageous in that a toner can be perfectly fixed. But, in such fixing method, a large quantity of energy is required, and it takes long time to heat the fixing apparatus up to a temperature at which toner can be fixed, and further the temperature of the periphery of the fixing apparatus becomes too high, which are drawbacks inherent to this fixing method.
The method for fixing a toner by applying pressure thereto is disclosed in U.S. Pat. No. 3,269,626 and Japanese Patent Publication No. 15876/1971 and is advantageous in many aspects. For example, only a very small quantity of energy is required so that economy of energy is possible; the air pollution is avoided; copying can be made immediately after the electric power is supplied to the copying machine; risk of a copy being burned is eliminated; high speed fixation is possible; and a fixing apparatus is simple.
In the pressure fixing method, however, the problems to be resolved are pointed out, for example insufficient fixability of a toner and offset phenomena of a toner to a pressure roller. Various studies and developments are made for the purpose of improving the fixability of a toner under pressure.
Heretofore, various methods for fixing a toner by applying pressure thereto have been proposed. For example, British Pat. No. 1,210,665 proposes a pressure fixing method which uses a toner comprising an aliphatic compound having 6-25 carbon atoms; Japanese Patent Publication No. 11493/1976 discloses a pressure fixing method which employs a toner comprising polyamide, a certain brittle resin, and polyethylene or polymethylene, Japanese Patent Laid Open No. 64931/1976 discloses a pressure fixing method which is applied to a toner comprising a low molecular weight polyolefine and terpene series resin; Japanese Patent Laid Open No. 137421/1976 proposes a pressure fixing method which is applied to a toner comprising a low molecular weight polyolefine or its copolymer, and an aliphatic hydrocarbon resin; Japanese Patent Laid Open No. 88228/1976 proposes a pressure fixing method which uses a toner comprising polyamide and polyester resin; U.S. Pat. No. 3,928,656 proposes a pressure fixing method which uses a toner comprising amorphous polymer having a weak bond strength of 2-30 Kcal/mol and exhibiting a second order transition point (Tg) higher than -20.degree. C. (Tg&gt;-20.degree. C.).
However, a pressure fixable toner for practical application has not been obtained which is advantageous in the following points: the toner can be easily prepared; it does not adhere to the pressure roller; it shows stable developability and fixability even in repeated use; it does not adhere or deposit to the carrier, metal sleeve and photosensitive member surface; it neither agglomerates nor cakes during storage; and it is excellent in storage stability.
Consequently, in the pressure fixing method practically applied at the present time, an electrostatic latent image which is formed on a zinc oxide-coated paper and developed with an electrically conductive magnetic toner is fixed by means of a pressure roller. This method uses a zinc oxide-coated paper as a substrate for a final image, and therefore it is totally different from the method for fixing a final image on an ordinary paper. Under existing circumstances, the pressure fixing method on an ordinary paper is not put into practice.
A toner comprising a soft material is excellent in pressure fixability, but various problems to be resolved are pointed out. For example, the soft material is difficult to grind to a toner powder, and the offset phenomenon of such toner to a pressure roller is liable to occur, and also such toner tends to adhere to the carrier and photosensitive member surface. Further problem is that such toner agglomerates and cakes during the storage thereof.
Another toner comprising a hard resin in excellent in chargeability and storage stability, and the hard resin is easy to grind to toner particle. However, such toner is inadvantageously very poor in pressure fixability for the reason that since the used resin is, in most cases, hard as compared with the cellulose fiber forming paper, it is merely crushed into paper in applying pressure thereto, but is not caught in the fiber.
Further, as for the known capsule toner of pressure fixability, when the core material is formed from a soft material exhibiting good pressure fixability the soft material gradually adheres onto a pressure roller during repeated practice of the pressure fixing method, which inadvantageously causes the offset phenomenon of the toner and the winding phenomenon of the transferring paper around the pressure roller. On the other hand, when these phenomena are avoided, the pressure fixability of the toner becomes deteriorated. Under circumstances, a practical capsule toner has not been obtained as yet.
Recently, an electrostatic latent image is developed with a developer of one component system in which magnetic fine particles is contained in a toner without carrier particles. In this case, however, the binder resin for the toner is required to have sufficient dispersibility and adhesion property to the magnetic particles, and further the toner should show good impact resistance and fluidity. Therefore, it is considerably difficult to make those properties compatible with the pressure fixability.